1. Field of the Invention
The invention relates to a display control apparatus and, more particularly, to a display control apparatus for a display apparatus having a display device which uses, for instance, a ferroelectric liquid crystal as an operational medium to display and update and which can hold a display state updated by applying an electric field or the like.
2. Related Background Art
Generally, an information processing system or the like uses a display apparatus as information display means which performs a visual expressing function of information. A CRT display apparatus is widely known as such a display apparatus.
In the display control in the CRT display apparatus, the writing operation of a CPU on the system side to a video memory serving as a display data buffer and the reading and displaying operations of the display data from the video memory are respectively independently executed by, for example, a CRT controller.
In case of the display control of the CRT as mentioned above, the writing operation of the display data to the video memory to change the display information or the like and the operations for reading out the display data from the video memory and displaying are independently executed, so that there is an advantage such that there is no need to consider the display timing or the like in the program on the information processing system side and desired display data can be written at an arbitrary timing.
On the contrary, however, a whole volume of the CRT is large because in particular, a certain degree of length in the thickness direction of the display screen is necessary, so that it is difficult to miniaturize the whole display apparatus. Consequently, when the information processing system using such a CRT as a display is used, degrees of freedom, namely, degrees of freedom of the installing location, portability, and the like are lost.
A liquid crystal display (hereinafter, referred to as an LCD) can be used as a display apparatus which can eliminate the above drawback. That is, according to the LCD, miniaturization of the whole display apparatus (particularly, the apparatus is made thin) can be realized. As such an LCD, there is a display (hereinafter, referred to as an FLCD: FLC display) using a liquid crystal cell of a ferroelectric liquid crystal (hereinafter, referred to as an FLC). If is a feature of such an FLCD that the liquid crystal cell has a preservation performance of the display state for the supply of an electric field. That is, the liquid crystal cell of the FLCD is thin enough and molecules of the elongated FLC in the liquid crystal cell are oriented in the first or second stable state in accordance with the applying direction of the electric field and maintains each orientation state even when the electric field is extinguished. The FLCD has a memory performance by the bistability of such FLC molecules. The details of such an FLC and an FLCD are disclosed in, for instance, Japanese Patent Application No. 62-76357.
Although the FLCD has the memory performance as mentioned above, since a speed regarding the display updating operation of the FLC is relatively slow, there is a case where the FLCD cannot follow a change in display information such as movement of a cursor, character input, scroll, or the like in which the display content must be quickly rewritten.
In the FLCD having such contradictory characteristics, in accordance with those characteristics or in order to compensate those characteristics, various driving methods of displaying can be considered. That is, with respect to a refresh driving method of sequentially continuously driving scan lines on the display screen in a manner similar to the case of the CRT or other liquid crystal displays, the driving period has relatively a surplus in terms of the time. In addition to the above refresh driving method, a partial rewriting driving method of updating the display state of only a portion (line) corresponding to the change on the display screen or an interlace driving method of driving while thinning out the scan lines on the display screen can be performed. A tracking performance for a change in display information can be improved by the partial rewriting driving method or interlace driving method mentioned above.
On the other hand, if such an FLCD and a CRT can be used with a compatibility as a display apparatus of the information processing system, a flexibility of the system increases and its value can be raised.
Since various kinds of driving methods can be performed in the FLCD as mentioned above, it is important to select the proper driving method in accordance with the display information (character, graphics, natural image, etc.) or a display changing mode (still image, moving image, scroll, etc.).
For example, in the scroll display mode among the display changing modes, it is necessary to perform a rewriting operation (display change) at a relatively high speed on the whole display screen and it is necessary to assure a picture quality such that display information such as characters or the like which are displayed can be clearly recognized in this instance. It is, therefore, necessary to select the optimum driving method in the scroll display mode. For this purpose, however, the setting of the scroll display mode must be certainly recognized on the display control apparatus side.
In case of using the FLCD as a display apparatus of the information processing system while keeping the compatibility with the CRT, however, the CPU on the system side merely transmits the display data regarding the display change and its address to the display apparatus side. Consequently, there is a problem regarding how to certainly recognize the scroll mode on the basis of the transmission of the display data and address.